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Why is the ball no longer centred in a multi-engine airplane with inoperable engine?
I'm currently working on my PPL, and whilst trying to understand fully how the turn coordinator works, I got down a rabbit hole that makes me question my understanding, and that I can't dig myself out of, namely:

Why, in a multi-engine airplane with one engine inoperable, is the ball no longer centred when the aircraft is flown coordinated?

There are multiple resources (such as this Wikipedia article), which reference that fact, but I couldn't find anything that actually explains why that is. I don't quite understand why the ball would suddenly sense aerodynamic forces differently when an engine fails. If the centripetal force is equal to the centrifugal force, it will be equal whether the engine is out or not, and the ball has no knowledge of its state (or even what an engine is).
I would very much appreciate a complete, from the ground up explanation that doesn't assume any knowledge of multi-engine operations. I have a basic understanding of aerodynamic forces involved in single-engine powered aircraft flight, including the concept of coordinated flight, as well as the typical steam gauge instruments, including a turn indicator/coordinator.

With asymmetrical engine thrust, there is no such a thing as level coordinated flight. You will have either sideslip, or a mismatch between turn and bank, or more likely both.

What is this "ball" you speak of?

@rob74: the ball is the colloquial name for the inclinometer in a turn coordinator

Asymmetric thrust means now you have more thrust on one side than the other. A yawing moment results.
You want to balance this yawing moment in order to still fly straight. This means the airplane needs to produce an opposing yawing moment somewhere else. The most suitable part to do so is the vertical tail surface, which will do this by producing a side force by means of rudder deflection.
With that side force, the airplane would accelerate sideways, so you need a compensating side force which balances that of the vertical tail. This can be achieved with a slight bank angle, so your wing creates this compensating side force.

Now you fly at a sideslip and a nonzero bank angle, but still on a straight line. This makes the ball move away from the center position.
Note that a similar effect makes Sikorsky-type helicopters hang sideways in hover: In order to compensate for the side force created by the tail rotor to counter rotor torque, the rotor disk needs to be tilted sideways in order to create this countering side force. Otherways the hovering helicopter would drift sideways. From the tail rotor lever arm and the amount of tilt you can directly read how efficiently the rotor converts torque into lift.

Fascinating that computer controls may eliminate the "snaking" tendency which forced early (subsonic) jet designers to use a large tail.  We may get that Horton flying wing yet in commercial aircraft.

So much more efficient to put the trimming, control and stabilizing surfaces at the end of a long moment arm using a tube.  Then you can make the tube really big to put stuff in.  Packaging the volume lengthwise in a tube fuselage means way less frontal area than the same volume put cross-ways inside a wing, and without all the stability and control issues.  There's a reason that 99.9% of airplanes are NOT flying wings beyond special niche applications, and it's not really control issues; it's the packaging issue of having all the interior volume oriented spanwise.

@JohnK Exactly, and this is even more so once the tube containing that stuff is pressurized. Adding a tail saves surface area and structure overall.

@John K: OTOH everybody gets a front row seat.

(Note: the first two paragraphs of this answer were aimed at the original title of this question, which was "Why does the ball no longer point straight down in a multi-engine airplane with inoperable engine?")
First let's address a misconception that some readers may take from your title.  In linear, constant-heading flight in any aircraft, "coordinated" or otherwise, the slip-skid ball always "points" straight down in relation to the earth.  If you have a conception that in linear, constant-heading flight  in a multi-engine aircraft with a failed engine, the ball is not pointing "straight down" toward the earth, then you have a misconception-- regardless of whether or not we are applying the optimal rudder input and bank angle to keep drag to a minimum.
But "straight down" is not the same as "centered", if the aircraft is banked.  More to the point, regardless of bank angle, and regardless of whether the intended flight path is a straight line or a turn, it turns out to be not optimal to keep the slip-skid ball exactly centered, when dealing with a failed engine on a multi-engine aircraft.
Now moving on to your actual intended question as I understand it-- I'm assuming that the real essence of your question is "why is not optimal to keep the slip-skid ball centered,  in the case of engine failure in a multi-engine aircraft?"  Or to put it another way, in this case, "why is the ball not centered in 'coordinated' flight?"
You need to start by asking yourself what is the meaning of "coordinated flight".  We could define "coordinated" in several different ways, one of which would be that "the ball is centered".  But if by "coordinated" we mean that the the fuselage is pointing straight into the relative wind, not yawed to one side-- i.e. if we mean that a tuft of yarn ("yaw string") mounted in the middle of the windscreen would be centered, not blowing to one side-- then we don't always want the ball to be fully centered, especially in the case of a failed engine on a multi-engine aircraft.
What's going on here is that when the fuselage is fully streamlined to the relative wind and not yawed to expose either side of the fuselage to the airflow-- the optimum situation for minimizing drag-- then the ball cannot be fully centered, because the rudder itself, which is strongly deflected to compensate for the failed engine, is generating a significant aerodynamic sideforce, and therefore a significant acceleration, toward the failed engine.  Compensating for this sideforce by banking slightly toward the good engine stops the flight path from curving, i.e. stops the aircraft from turning, so that the aircraft's heading remains constant, but doesn't affect the position of the ball, so the ball continues to be off-center.
If we applied enough rudder pressure to fully center the ball, this would mean that we were exposing the side of the fuselage (the side nearest the failed engine) to the airflow, thus creating an aerodynamic sideforce that exactly cancels the sideforce from the deflected rudder.  (And in this case we could hold heading with the wings level-- there would be no turning tendency.)  This is not optimal--drag is greater than it needs to be.
Note that in both cases the torque from the deflected rudder is cancelling the torque from the thrust asymmetry.  In the latter case, the slightly greater rudder deflection is actually creating a little extra torque, that is used to keep the fuselage and vertical fin pointing slightly sideways in relation to the relative wind and airflow-- again, this is not optimal.
In theory this happens any time we deflect the rudder for any reason.  If we want to keep the fuselage streamlined to the airflow, and the rudder is deflected at all, the ball should not be fully centered.  Rather, we should apply slightly less rudder than would be needed to fully center the ball.  Otherwise we are exposing the side of the fuselage, the side opposite the deflected rudder, to the airflow.  But normally this effect is trivial enough that we don't worry about it.  Not so when the rudder is strongly deflected to counteract the torque from a failed engine.
We could imagine some other system of yaw control-- such as the "clamshell" split ailerons on the B-2 Spirit-- that would create plenty of yaw torque, but no significant aerodynamic sideforce.  In this case, if there is a thrust asymmetry, to fully align the centerline of the aircraft with the direction of the flight path, relative wind, and airflow, the ball should simply be centered, not left deflected to one side.1
Maybe it would help to explain it this way: one way of looking at things is to say that you should use the rudder as needed to hold the heading constant, and then, while continuing to hold the heading constant, bank toward the good engine enough to move the ball slightly off-center (one-half diameter is often recommended).  But another way of looking at things is to say that you should apply rudder as needed to bring the ball nearly but not fully to center (again one-half diameter off center is often recommended), so that the fuselage is fully streamlined to the airflow, and then, while keeping the ball in that position, bank as needed toward the good engine to stop any turning tendency and hold the heading constant.  In theory, you'll end up in the same place with either method.  Granted, in the real world, there are reasons why it's very important to immediately counteract any yaw toward the failed engine, so the first method-- prioritizing holding the heading constant-- may be better.  But thinking through how the second method would work, may give you a better understanding of what is really going on in terms of aerodynamic forces and torques.
PS-- I guess I'm starting from an assumption here that you understand the difference between sideforce and torque.  I'm also starting from an assumption that you understand what the slip-skid ball (inclinometer ball) really indicates.  The slip-skid ball  responds to the net sum of all lateral (sideways) accelerations acting on an aircraft, excluding the lateral acceleration component due to gravity. And this is using  "lateral" or "sideways" in relation to the aircraft's own reference frame-- that's why banking doesn't affect the position of the ball in and of itself, nor does it generate any aerodynamic sideforce.  (Obviously it's a different story if we are also changing some other variables, for example if we are manipulating the rudder as needed to force the flight path to remain in a straight line as we vary the bank angle, in which case we'll be demonstrating a sideslip.)  For (much) more, see this related ASE answer: What does the balance ball actually indicate?
See also this related ASE answer: Why should you not turn in the direction of an inoperative engine?
And see also this related answer: How can using split throttles help when landing twins in crosswinds?
Closing note: note that this answer and at least one other posted answer are basically in agreement, except that this answer uses the word "sideforce" specifically in relation to the aircraft's own reference frame. By this convention, banking creates a horizontal (sideways?) force that keeps the flight path from curving (keeps the aircraft from turning), but does not actually create a "sideforce" in the aircraft's own reference frame, hence the bank angle does not directly affect the deflection of the slip-skid ball.  Also, the present answer, while possibly more verbose than absolutely necessary, is intended to encompass situations where the aircraft is not simply travelling in a straight line.
Footnotes:

But on the other hand, minimizing sideslip (centering the yaw string) is not that important, in terms of minimizing drag, in an all-wing aircraft like the B-2, with minimal cross-section as seen in side view.  Consider this quote from the 35th Wright Memorial Lecture by Jack Northop on the development of flying-wing aircraft: "for very long-range aircraft there is a valuable compensating advantage in being able to fly under conditions of asymmetrical power without appreciable increase in drag."  In such a case the pilot should simply make whatever control inputs most efficiently balance the aircraft in roll, with minimum drag penalty, and so long as the aircraft has efficient ailerons, minimizing sideslip (centering the yaw string) will be less important than in a more conventional aircraft.

You're right, I should've said "centred", calling it "straight down" is inaccurate and confusing wrt the substance of my question. I'll fix it.

@mathrick -- Sounds good --

Note that this answer and Peter Kampf's answer are basically in agreement, except that I'm using the word "sideforce" specifically in relation to the aircraft's own reference frame. Banking creates a horizontal (sideways?) force that keeps the flight path from curving (keeps the aircraft from turning), but does not actually create a "sideforce" in the aircraft's own reference frame, hence the bank angle does not directly affect the deflection of the slip-skid ball.

Also it appears that this answer, while obviously more verbose, might be better suited to understanding situations where the a/c is not simply travelling in a straight line.

The simple answer is your straight and level flight is achieved with 5 degrees of bank.
So the tube is not leveled anymore so zero forces would sit closer to the lubber line on the side of the engine that's being banked into i.e. working engine.
Try this on your cessna, sit inside ask a friend to tilt the wing up on one side and see where the ball sits.
I guess I should add that if your fly straight with no bank into the inoperative engine ball should be centered. That would mean a more agressive rudder input and also a higher Vmc speed.

Food for thought: my answer contains the statement (final paragraph) "hence the bank angle does not directly affect the deflection of the slip-skid ball".  Seems to conflict with this answer.  Is there a real contradiction here, or not?

My statement was made specifically in the context of a model where we are not necessarily constraining to the aircraft to be moving in a straight line (or stationary). Once we add the constraint that we are not travelling in a curve, i.e. we are travelling in a straight line (or stationary), then it's obvious that banking will affect the position of the ball.

But is that really an appropriate constraint in the context of flight? It's not like we're travelling down a railroad track after all. Removing that constraint leads to longer and more complicated explanations, but which may have deeper explanatory power as to what is going on on the most fundamental level.


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For electric aircraft, is MTOW = Empty weight + Maximum payload?
Electric aircrafts and most drones have simple powertrains, and their weight does not vary along the flight. After reviewing this thread I assume that:
MTOW = Empty weight (batteries incl) + max payload ?
I have reviewed some drone specs, and sometimes the numbers match, others don´t. Am I missing something?

At least conceptually, an electric aircraft might have optional batteries that could be installed or not depending on the required range. Those wouldn't quite be "payload," nor would they be "empty weight" either. A long-range mission on such an aircraft might use max batteries & be able to carry less payload weight than a shorter flight than only needed the installed batteries. In either case, takeoff weight would be limited by max certified takeoff weight, unless performance becomes more limiting (high altitude field, hot weather, short runway, etc).

https://en.m.wikipedia.org/wiki/Operating_empty_weight

A maximum payload may be specified to be less than tested MTOW minus empty weight in some cases, as a precaution against conditions that may reduce MTOW -- which depends on strip elevation, barometer, and temperature.
The thinner (higher and hotter) the air, the less a given amount of power can lift, in the case of a quad-copter or similar aircraft, or the longer it will take to reach takeoff speed and the slower climb will be, in the case of a conventional winged drone (electric or otherwise, though at least electric motors don't lose power with altitude).  Responsible drone manufacturers may (and should) specify a maximum payload spec that will keep gross weight below the least MTOW for foreseeable conditions.
There may also be cases where, for one reason or another, payload is based on testing conditions different from your flight configuration -- for instance, if you're using a heavier battery pack than the one used for testing, you'll have less margin below MTOW than the test condition.  Payload limits may also be specified below MTOW minus empty weight for reasons of center of mass location, payload restraint limitations, or other engineering reasons.

So it sounds like you're saying that the end result is, Max Takeoff Weight is unreachable if all other limits are met. In transport aircraft, the reverse is always the case: MTOW is not only reachable but could be exceeded (i.e. max fuel+max cargo+max pac) - thus MTOW places a limitation on the operation, and it must be observed. Along with many other limitations, such as performance requirements (which themselves may keep you well below the certified MTOW in a hot, high altitude case).

@RalphJ At this time, there are no all-electric transport aircraft.  The question spoke about electric aircraft and drones.  For drones, especially, payload specification may be specifically nerfed to avoid exceeding MTOW, because manufacturers hate having to explain to "users" that their drone won't take off today with the same load it lifted last Sunday, because it's too hot out.

There is a difference between MTOW and the actual weight of the aircraft and payload. MTOW, or Maximum Take/off Weight, is the amount of weight the aircraft is certified to carry on takeoff. 99.99% of the time, the actual weight of the aircraft and payload is lower than the MTOW. But I think you know this. I think what you meant was does the MTOW include the empty weight, or is it just maximum payload.
In that case, yes, MTOW does include both the MAXIMUM payload and the EMPTY WEIGHT of the aircraft (including electric).

How does your answer change in the case of an electric aircraft vs a conventional aircraft? That's the point of the question.

Oh didn't read it properly. But I don't think it changes much, does it?

@AirCanada001 This answers a different question... I'm inclined not to delete in the event you can edit it to address the specifics of electric aircraft.

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Are General Aviation tires filled with Nitrogen?
Long story short, I'm working on a concept for a book.
We all know why commercial aircraft have nitrogen tires, but my question is if GA aircraft have nitrogen tires, or are they just filled with helium, oxygen or any other gas? My theory is that commercial aircraft are heavier, so probably need nitrogen tires, whereas general aviation aircraft are much lighter, so wouldn't be needed to withstand as much surface friction, meaning it may not need a more heat-prone gas for the tires, like nitrogen.

See also Does Nitrogen inside commercial airliner wheels prevent blowouts on touchdown? for further discussion on why some aircraft tires are filled with nitrogen.

@Greg Hewgill: I've edited it. Thank's for the suggestion.

No worries. I mostly wanted to link to that other question because not everybody knows why nitrogen is used in the first place.

You would never put aviation grade oxygen in a tyre. Oxy and oils/greases when mixed causes combustion. Minimum distance these should get is 20m unless it's installed in an approved container (pressure cylinder or converter) on the aircraft.

Although I can’t speak for every GA pilot, the aircraft that I have flown have had their tires inflated from an ordinary air compressor. We have used handheld battery operated compressors, floor standing compressors, small-tank mobile compressors, and hand carried compressed air storage tanks.
The max speed that we actually use the tires is under 100 mph. The max weight that we carry on them is under 6000 pounds.

Just for further clarification, what aircraft do you fly?

SportCruisers; Cessna 152, 172, & 182s; Piper Cherokee, Warrior, Archer III, Seminole, Arrow, Apache, & Aztecs; Beechcraft Duchess; Robinson R22s

It comes down to what's available.  If your airplane is at a facility of some kind where there is a dry nitrogen bottle available to you, you will use that.  But you will only find those kinds of bottles at maintenance shops.  Most of the time you are using regular compressed air as Dean says.
I inflate the tires of my plane with whatever compressed air source is handy, to about 25-30 psi.  Small GA tires, which generally have inner tubes, lose significant pressure over time because of the small volume and the presence of oxygen which permeates the rubber more readily; a lot more than car tires, about 2-3 psi a month I'd say. Nitrogen would reduce the pressure loss a bit, but compressed air is cheap and the tires are pretty tolerant of the pressure variations, and they don't get very hot so thermal expansion isn't that big a deal.

“ It comes down to what's available.” - Interestingly enough, I can get my car tires filled with nitrogen at my local Costco any time I want. But, my local FBO fills aircraft tires with a handheld, battery operated compressor. My local flying club uses a 1-gallon mini-compressor. Doesn’t that seem a little backwards? I wonder if the FBO saves the nitrogen for their jet traffic?

Well, nitrogen in car tires is called "marketing" where there is money to be made on something completely worthless, from an actual real-world practical standpoint. because people hear that racing and airliners do it.

@John K: True.  And consider that if oxygen does diffuse out of the tires faster than nitrogen does, after topping up the pressure a few times, the gas in there is going to be pretty close to 100% nitrogen anyway.

Air is free, easy to pump into tires and works very well. Seems like a winner to me.

As made clear in the link provided by Greg Hewgill, there are valid reasons why commercial airliner tyres would want pure nitrogen.  The same reasons might apply to a racecar.  But for the majority of personal cars and GA aviation, plain old air, which is 78% nitrogen, seems just fine to me.

Does the altitude that airplane tires are subjected to affect the pressure loss over time?

There probably is some detectable difference, but not that you could tell in practical terms I would say.  A GA tire at 25 psid on the ground will be about 30 psid at 10000 ft.

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Why do flaps retract?
Why do flaps retract? I know, it might seem obvious at first, because of drag, but instead of retracting the flaps, can't they just make the flaps flexible enough to get into a reduced drag position, and act like an extended wing?
Maybe there is a reason we should look into this technology, since it could really help out in terms of climb rate. Or, was this already looked at, but engineers decided something was wrong about it, and threw the idea away?
And by the way most of the answers involved automatic retraction of the flaps, even though I never said anything about making the flaps being automatic. Just a disclaimer.

On an airliner flying at 150 knots, how would your flap system distinguish between take off and landing configuration? How would you expect a pilot flying in gusty conditions at low level to handle a wing that changes its configuration in response to every gust?

You need different flap settings for different conditions and scenarios. CACC has mentioned a couple. Consider just a small GA aircraft’s (like a C172) different needs. You need no flaps for normal takeoff, and 10° for soft and/or short field takeoff. You may need 20° in gusts and heavy crosswind landings. Yet you need 30° for a normal landing, and 40° for soft and/or short field landings. Some pilots like to retract the flaps on rollout to increase braking efficiency. Others prefer to wait until fully exiting the runway. And, how would you resolve asymmetric flaps if you can not retract them.

The mechanism you describe cannot take into account the change in aspect ratio of some type of flaps that extend backward (e.g. fowler flaps)

Why the downvote? What is so bad about an out-of-the-box question that would deserve a downvote?

@PeterKämpf thanks.

@L'aviateur. J'assume que tu parle francais. But maybe not. Anyways,  Ive never seen flaps that dont before.

What link with france?
I was wrong ... confusion

I'm sorry. It's just that your name is french and also you posted a question that had a diagram in french. But, I guess not?

Flaps retract in order to reduce wing area. This has several advantages when flying fast:

The higher wing loading (weight per lift-producing area) reduces gust loads. When hit by a vertical gust, the angle of attack increases suddenly, and so does lift. If this happens at high speed (more precisely: At high dynamic pressure), the lift increase might overstress the wing. A reduced area will only produce so much extra lift before the wing stalls, which limits the extra lift from gusts.
Less exposed area also means less friction drag. Your flexible flap proposal would still leave both sides of the flap exposed, adding more surface for friction drag. Put another way, the higher wing loading shifts the lift coefficient up which generally improves L/D at high speed.
Retraction closes the gaps between wing and flap sections. While those gaps are important to maximize the lift coefficient for the lowest possible landing speed, they increase drag at all lift coefficients.

The retraction mechanism is heavy and increases maintenance cost, but it is worth it.

Very detailed yet understandable answer. Thanks for explaining it so well.

A high-bandwidth, flexible flap with a high-bandwidth flow sensor, e.g., gust sniffer, could be used to mitigate gust loads through a closed-loop gust load alleviation. If the flap were further conformal then it could mitigate the friction drag. This is the ultimate objective of the AFRL's effort to develop a compliant trailing-edge flap.

Believe it or not, "flexible" control surfaces are a secret to competition throwing gliders, where less pitch up is needed for the throw (to keep it from looping at high speeds) and more for staticly stable gliding once it slows down.  These uncontrolled gliders, generally made of  paper, have their elevator trim as a surface that flattens at high speed and springs up when aerodynamic forces are lower.
On manned aircraft, there are several different flap settings a pilot may wish to use for extra lift and/or extra drag.  These positions must be rigidly held to control the aircraft.  Having them move without pilot command would be unpredictable, there for, unsafe.

Flaps are used to control the amount of lift and drag required during flight. e.g. during take off and landing required lift is much higher than maintaining a constant altitude at cruising speed (the high drag during landing also helps to reduce speed), so the flaps are extended during take off and landing.
If the flap is flexible so that it will adjust automatically to the minimum drag position then the pilot will not have control over the lift generated. Even though there are other ways of controlling the lift and drag, using flaps is one of the fastest way, which gives much better control to the pilot.
Flaps are necessary to control the airplane efficiently during take off, landing, turbulence, bad weather and several other situations.

You may develop your second paragraph. Why is it better? what are the pro/cons of other solutions?

Plain flaps are as you suggest, just sections of the trailing edge that can be turned down. That helps slow the plane for landing and gives a little extra lift, but for a plane to fly slowly and safely you need as much wing area as possible.
So next up the scale are split flaps, which are just the underside section of the trailing edge. When these lower, the upper part remains so wing area is not reduced.
More effective still are flaps which extend backwards, actually increasing the wing area, as well as downwards. Leaving these flaps extended and just raising them in level flight is not viable, as they create excessive drag and are subject to excessive aerodynamic forces when the plane tries to accelerate to cruising speed.
Flexible flaps are also not a viable solution, because the air at the trailing edge is always turbulent to at least some degree and movable surfaces in this position are prone to flutter, in which they vibrate rapidly up and down. This leads both to undesirable handling effects and, ultimately, structural failure. Flaps have to be held rigidly in place when retracted.

Thanks for sharing other kinds of flap designs with me. The whole reason I asked this question in the first place was to make something a little bit outside of the box. I'm happy you're doing the same.


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How do RC airplanes create thrust without using propellers or rockets?
I haven't wondered how RC planes create thrust until now. I know some literally use rockets, but what about the small ones, that don't use propellers? How in the world do they manage to create thrust in the air without propellers, or rockets?

Also, how do they control aileron? Does an RC plane have a rudder? Do RCs have to be registered with local aviation authorities (if in Canada)?

Google electric ducted fan.  Many RC model planes fly just fine with no rudder-- especially if wingspan is not too large and flight speed is not too slow.  Laws regulating RC models vary from country to country and are getting ever more draconian-- in the US RC model flying as we know it will essentially be illegal if new regulations proposed by FAA this year actually end up going into effect.  Google "Remote ID NPRM".

Your last section should be asked separately. But the first couple may be considered off topic, and the last has answers, depending on jurisdiction.

@fooot: There's a difference between an RC and a drone.

RC hobbyist quadcopters and rc airplane are all "small Unmanned Aerial Vehicles" in the eyes of the FAA here in the USA whether they are flown the normal way or by a camera onboard or autonomously.  Of course some of that is illegal if you don't have someone watching the thing with their unaided  eyes also.

@quiet flyer: you know you can edit it yourself, right? Anyways, I fixed it.

You have several questions that might be better asked separately. For the question about regulations, which country or regulator are you asking about?

They use jet engines for RC models like the ones sold here, for example https://www.chiefaircraft.com/radio-control/turbine-engines/jetcat.html

You should know that there's now Drones.SE for questions like this.

They create thrust the same way that the full size ones do.

In the case of a piston engine type aircraft they have either have a miniature version of a piston engine, or an electric motor which turns the propeller.
In the case of jet engine, like your picture, they have miniature versions of a jet engine.
Sometimes, the jet engine in an RC aircraft is actually a propeller made to look like a jet engine.

Also, how do they control aileron

Usually by using servos to pull cables and pullies - essentially like aircraft used to before fly-by-wire systems were introduced.

Does an RC plane have rudder?

Some do, some don't. Its the difference between 2 and 3 axis controls. Anything like the one in your picture almost certainly do.

Do RCs have to be registered with local aviation authorities

Sometimes, It depends on jurisdiction and and often weight.

@PerlDuck wow not cheap toys are they now!

Indeed. But impressive they are. Btw. I removed my comment and put it below the question so the OP gets notified.

I thought 2axis controls removed ailerons, not rudders, mainly because of secondary effects of rudder (induced roll)

@ManuH There are some of both.  I've flown many hours of RC with rudder-elevator and rudder-elevator-throttle, but most of my time with aileron-capable RC I've ignored the rudder,effectively flying aileron-elevator-throttle most of the time -- and there are models that simply have a fixed fin, no rudder at all (more common back when a 4-channel radio rig cost twice what a 3-channel one did).


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Why do turbofans go faster than turboprops?
Can someone explain why turboprops are less effective in terms of thrust than turbofans? But at the same time, why turboprops burn less fuel? For example, the Bombardier Q400/De Havilland Dash 8 has a fuel burn of 2.79L/seat/100km, whilst the 737 MAX 8 with the same number of engines burns 2.93L/seat/100km, but the Bombardier has a max. cruising speed of 360 knots and the Boeing plane can go up to 526-527 knots, even though it's heavier.

Most, if not all, turboprops can put the propellers in beta and generate reverse thrust.

Because they are attached to faster airplanes?

To generate a given thrust, a turbofan accelerates less air but faster than a turboprop. This means that turbofans can fly faster. However, it is more efficient to accelerate a greater mass of air slower, so turboprops are more efficient and that translates into lower fuel burn, as less power has to be generated.

It’s all dependent upon what operating speeds your aircraft will be in.  Propellers can move a great volume of air at slow speeds but accelerate the gas to a lower exit speed than does a turbojet or a turbofan.  This is the principal reason that a helicopter is the most efficient means of VTOL flight out there for a heavier than air aircraft.  But as speeds increase, the the specific impulse drops and the propeller efficiency drops off with it.  At around 400-500 mph, the specific impulse of a high bypass turbofan becomes superior to the turboprop, as, while it moves less air than a turboprop, it can accelerate the smaller air mass to a much higher exit velocity.  This trend continues through low bypass turbofan engines, turbojets and then ramjets.
And as pointed out turboprops have a beta and reverse pitch range which can produce reverse thrust.

Turboprops are less fast because of the mechanical constrainsts at the blade tips,efficiency and noise near the speed of sound.
For reverse thrust:yes
related :
How does a turboprop engine keep running during thrust reversal?


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Why single engine turbine helicopters can operate on higher altitudes than multi engine helicopters?
Probably you knew the example where H125 landed on Mount Everest (8848 m), and H145 highest landing was on Aconcagua (6962 m), almost 2000m difference.

The helicopter landing on the mount Everest was a special version of the H125, so the comparison is a bit unfair

@shophit "Delsalle used a virtually standard version of the Eurocopter, only removing unnecessary elements, such as passenger seats" - your link. I'd argue that's not a special version, nor even a modified one. Afaik those seats are removed more often, e g. for more cargo capacity when needed. Standard version, special configuration perhaps...

@DaniëlvandenBerg: well 120kg less is 10% of the empty weight, which is a lot less. "Unnecessary elements" means also equipment not necessary for that particular mission, not only a couple of seats. So it was definitely a special configuration not flying normally. Just out of curiosity, a "special" version (this time with 200kg less equipment) reached almost 13'000m height.

Engines are heavy.
There is no market for a high altitude helicopter.  Airliners fly high up because it lets them reach the destination quickly and cheaply.  Helicopter buyers want to fly slowly near the ground.  They buy a two engine helicopter to improve performance when an engine fails or to increase lift capacity.  They do not buy a second engine to fly higher.
Buyers of heavy lift helicopters, which have multiple engines, want the higher power of multiple engines, but the helicopters are shaped to hold cargo.  That's not the best shape for flying high.


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What is the brace position in light aircraft like a Cessna?
There is nothing to grab onto in the front, except the yoke, and it's not possible to bend over because, again, the yoke is in the way.

In light aircraft I was always taught to grab whatever is soft and cover your face/head with that. Mostly that applies to passengers only (as the pilot will likely be busy attempting to fly). There is no "brace position" analog in GA like there is in commercial, but if you are sitting in the back you can assume a similar position.

I must admit my experience with Cessnas are few and far apart (as well as temporally far), but the small planes I've flown in have had proper seatbelts with shoulder straps.
In such planes the brace position would be, after yanking the straps a little tighter, to sit as straight as possible. The seatbelts will keep you from hitting anything.
Quite frankly I'd not feel comfortable/safe flying in a small plane with only the lap belt...

I believe the lap belt is intended to help them identify your corpse rather than for protection

Much in the same way the second engine in small twins is there to take you to the crash site.

The first thing a new owner of a plane with only lap belts should do is install shoulder harnesses.

But you'd feel safe in a large aircraft with only a lap belt?

@acpilot - Shoulder harnesses are even better. They allow more of the corpse to be located.

CAIUS - a professionally flown, dual crewed turbojet is not the same as a GA piston flown by JoePilot.  Feel free to post a question if you are really unclear on it.  RICHARD - shoulder harnesses prevent you from breaking your skull on the panel.  Studies show that people with broken skulls are less likely to survive a forced landing that people without broken skullls.

@caiusjard yes, for starters they are flown by professionals, maintained to high standard, and are not older than me. And I am picky when it comes to flying in larger aircraft too, not all airlines qualify, best not mention any here...

Sounds like your perception of safety is little to do with the lap belt, tbh..

Explain please.

In the absence of a shoulder harness, the only thing you can do is lean forward and cross your arms on top of the panel, and rest your forehead on your arms (to the extent that you are tall enough to clear the yoke while doing it). It's better than sitting up and being propelled into the top of the panel.  You're still going to get hurt, but not quite as bad.
This is why the adoption of shoulder harnesses was such an important safety improvement in light aircraft in the 70s.  Shoulder harnesses reduced the head and chest injuries in crashes by a massive amount.  They made many serious injury accidents walk-away accidents, and fatal accidents survivable.
Hopefully the plane you are flying in has a shoulder harness, and if you have half a brain, and/or your pilot is halfway competent, you are wearing it properly.  Protected by the harness, the main thing is protecting your arms from flailing around by crossing them in front of you and hanging on, or possibly (what I would do) reaching down to your pants belt and hanging on to that for dear life to keep your arms from flying around (if you're just the passenger).
In any case, wearing a harness, you can expect to come through any crash that leaves the cabin intact with survivable injuries, like broken limbs.
When I bought my '68 Cardinal in the late 80s, it only had lap belts, and the first thing I did was install shoulder harnesses.  I dreaded having to ride or fly airplanes that didn't have them.

I don't have an opinion (meaning I'm not disagreeing or agreeing), so I'm wondering if there is any reference for the claim ending the first paragraph: You're still going to get hurt, but not quite as bad.

No I don't have a reference. When I learned to fly in the mid 70s there were lots of articles in the aviation press about shoulder harnesses, which were kind of a new thing just being introduced on production airplanes in the early 70s.Recommendation in the absence of a harness was to place your head where it was going to end up anyway, top of the panel, using your forearms as cushions, rather than letting it slam into it. It's a small mitigation to be sure. If you sit upright with just a lap belt, you are slamming face first into the panel, and you normally won't have any other padding handy.

Thank you for the further details and clarification John.

The logic in resting, say, ones head on something that it's gonna hit anyway is, that when the plane impacts ground it will not stop at once (with infinite deceleration), it will take fractions of seconds to come to a stop, and if your head is supported on the structure, the deceleration will be the same for the head and the structure. If you sit up straight with only lap belt, during impact your head will keep moving at the initial speed, and the plane will have time (those fractions of seconds) to decelerate before your head hits it. Therefore the impact will be much harder.

As you mention bending, I assume you are in the case of a lap belt. There is not much to do, except using airliners techniques. They include not stretching arms and not crossing forearms.
Lap belts in aircraft are effective against turbulence, less in case of diagonal impact. Contrary to cars, there are no airbags to prevent head from hitting panels or walls.
As commented in other answers, switching to a harness, when possible, makes sense.
From UK CAA Care of Passengers in General Aviation Aircraft

With a shoulder harness (left):

For passengers seated with a shoulder restraint, the straps should be adjusted tight or if fitted with an inertia reel, the mechanism should remove any slack in the belt. The brace position simply involves
placing the chin down against the body.

Facing rearwards (center):

If facing rearwards with a headrest or bulkhead behind you, rest you head back against it. Arms and hands can be secured by placing the palms underneath the legs or crossing arms in front of the body. Feet should be flat on the floor. Do not tense the body for
impact since a rigid body is more likely to be injured. Remove headsets before impact.

With a lap belt (right):

If only a lap belt is available, a more airline style brace position should be adopted, with head down towards the knees and hands placed one over the other (fingers not interlocked) on top of the head.

Additional instructions from ICAO for airlines (Doc 10086, Manual on Information and Instructions for Passenger Safety).
Acceptable:

Avoid crossed forearms and head tilted backward:

The passenger should avoid having the head tilted backward, that is, the neck should not be extended, but should be bent forward to reduce the risk of injury to the neck and/or larynx. The passenger should not rest their head on the crossed forearms, which risks fracturing both forearms.

Avoid upright positions:

Passengers should avoid upright positions, as their head may hit the surface in front during the secondary impact. Passengers should avoid stretching out their arms or legs and pressing them against a surface in front of them.

A harness is much more safe than a lap belt. For more information, see Are the current seat belts used in commercial airlines safe enough?. Lap belts exist because they are easier to use, and therefore might be safer than a harness not used correctly. Attendant seats have harnesses because attendants are trained to use them.

The C172N flight manual might be a good yardstick for what a brace position could be in the event of a forced landing.  In the amplified procedures section of Section 3 Emergency Procedures, it states under forced landings:
Prepare for ditching by securing or jettisoning heavy objects locat-
ed in the baggage area and collect folded coats for protection of occupants' face at touchdown.
Also item 8) under the ditching checklist states
(8) Face -- CUSHION at touchdown with folded coat.
A possible brace position for a passenger in such an airplane may be similar to a crossed arm position in a commercial aircraft, with a folded coat or pillow placed over your arms to protect the face.
For front seat passengers, arms could be rested on the tip of the glare shield prior to touchdown, though care would be required to prevent being struck in the sternum by the yoke in the event of a violent impact.


text:
Do commercial aircraft carry personal weapons?
Even though airport security is very strong nowadays, commercial flights are still at risk of hijacking. Alternatively, passengers (perhaps mentally distressed) could provoke damage to crew, passengers or infrastructure.
Do commercial aircraft carry personal weapons for their crew to use? This could be either fire arms or non-lethal weapons (like batons, pepper spray, Taser, etc). On the one hand, it makes sense they do carry some form of deterrent. On the other hand, in the wrong hands these weapons could create more danger than if not present. Do we know something about this?
Online I read about planes carrying an axe, but this is not much of a deterrent since it is quite hard to use properly and effectively (I imagine). There is an old discussion here in the US context, which might suggest this is an ongoing debate.
PD: perhaps a firearm shot could affect the fuselage and put the whole aircraft at risk, so non-lethal weapons would make more sense.
PD2: this Q is broader than the suggested duplicates. It is not only about pilots. The existing answer confirms its broader nature.

which legislation are you asking about?

@Federico Sorry, the Q is general. I imagine answers could depend on jurisdiction, which is in itself interesting. Do not want to restrict scope ex ante

The second article you linked is your answer (at least for US). The Federal Flight Deck Officer program exists (and is not an ongoing debate as you say), so there are armed pilots on US airline flights.

@mins useful or not, many countries seem to think it worthwhile: https://en.wikipedia.org/wiki/Sky_marshal

@Jamiec: This is different, police officers in the cabin, not armed crews. But you have a point.

Well, question never really specified. "Do commercial aircraft carry weapons for the crew?" simple answer: "Yes, concealed with a crewmember trained on how to use it". The fact is no airline or country really publicises many details.

"perhaps a firearm shot could affect the fuselage", it's unlikely this would be disastrous, but as soon as there is a weapon aboard, it might also be used by criminals if they can seize it. The main problem is here.

Does this answer your question? Are pilots armed? See also this question.

Cockpit crewmembers carrying handguns applies only to the US.  In any case, hijackings have largely stopped for two reasons: reinforced cockpit doors, and the knowledge by the pax that a hijacking is likely to result in death and they have nothing to lose by trying to overpower the hijackers. This has changed the entire dynamic because before 9-11 a hijacking was usually assumed to result in a diversion and landing somewhere, so passivity was the wise option.

Check this out... title of the video is, "An unruly Air Marshal forces emergency landing"  https://www.youtube.com/watch?v=esZVVXXZaLc  Something to think about, how weapons around can cause troubles even if in the hands of trained professionals.  Everybody is human.

Many years ago there was research into mechanisms of having a sleeping gas added to the environment within the cabin in the event of a hostile takeover.  I'm not sure if that research ever led to the designs being implemented.

When I saw the title of this question, I thought for a moment of a sapient aircraft carrying personal weapons for its own use.

Did you notice any old discussion in the US context could only be relevant to domestic, not international flights? Does that not change the scope of the Question?

The second amendment to the constitution of the United States reads-- "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."-- commercial aircraft are not "people", and therefore in the USA, airplanes do not have the automatic right to "carry" weapons, and are generally not allowed to do so.  Except in Texas, where anything goes.

Many countries and/or airlines have a sky marshall program where a specially-trained law enforcement (or possibly military) person is on board with weapons used in case of hijacking.
Some examples:

United Kingdom aviation security is handled by SO18: https://news.bbc.co.uk/1/hi/uk_politics/2590309.stm although it is unclear whether SO18 do any more than patrol the ground at Heathrow & London City airport.
US has the Federal Air Marshall Service which oversee the Federal Flight Deck Officer (FFDO) Program
El-Al (Israel's flag carrier) has since its inauguration always carried armed personnel: https://news.bbc.co.uk/1/hi/world/americas/2097352.stm

It should be noted that every country and/or airline is pretty tight-lipped about whether they do or do not employ sky marshals, probably for fear of affecting the operational usefulness of such roles. I don't think we can know for sure exactly where armed personnel are onboard!

Should be said that BBC link is from 2003 and I don't think I've once heard about a UK "Sky Marshal" since. I'm sure they conduct specific intelligence-led operations, but I don't think the UK actually has a routine sky marshal program like the US does

@Dan Oh I agree, SO18 is only really responsible for protecting the airports at Heathrow and City. But once upon a time I think they may have considered putting armed marshalls on aircraft (and maybe they do - they just keep it secret)

Just to highlight the complexity of the legal matters: Air Marshals: The Need for Legal Certainty, with quite surprising cases (e.g. El Al Flight 432 and Dawson's  Field)

Of course, the Brits keeping the confusion level up about whether they have sky marshals or not helps keep certainty level down for the bad guys. Thus adding a touch more uncertainty to the bad guys' hopes of success.

Another reason, especially on international flights, is that bringing those weapons into another jurisdiction may well break the law. By treaty exceptions can be negotiated for law enforcement agents and their weapons. But that won't work for airline employees.

As of March 2009, some US commercial pilots were trained in the use of weapons and were allowed to have them on board
Source: https://www.factcheck.org/2009/03/guns-for-pilots/
The program started, not surprisingly, soon after 9/11
https://www.nytimes.com/2002/07/10/politics/house-passes-bill-to-allow-airline-pilots-to-carry-guns.html
It looks like it's still in force (from March 2018):
https://www.bbc.com/news/world-us-canada-43377461


text:
Re old (1992) US VFR sectional charts-- when was the thin dashed magenta line first used?
When did the thin dashed magenta line, currently used to illustrate Class-E-to-surface airspace, first appear on the legend of US VFR sectional charts?  I'd like to narrow down the date as precisely as possible. Currently I'm aware that this marking DID appear on the legend of a sectional chart (Hawaiian Islands) effective November 12 1992, and did NOT appear on the legend of a sectional chart (Bethel Alaska) effective August 20 1992.  Please post the "effective date" of any US VFR sectional chart that you are aware of before November 12 1992 in which the thin dashed magenta line does appear on the legend, or the "effective date" of any US VFR sectional chart that you aware of after August 20 1992 in which the thin magenta line does not appear on the legend.
Note that on the legend of the November 12 1992 chart, the thin dashed magenta line is identified not as depicting "Class E airspace", but rather as depicting "Control Zone at airport without control tower and Control Zone extension without communications requirement".  The term "Class E" didn't show up on legend of US VFR sectional charts until 1993.

all instances of "thin magenta line" should read "thin dashed magneta line"

It was in place in the US already when I started flying in April of 1994. But hadn't been for very long at that point. Long enough tho to make into the text books.

Re "Note that on the legend of the November 12 1992 chart, the thin dashed magenta line is identified ... as depicting "Control Zone at airport without control tower and Control Zone extension without communications requirement"." -- it's worth nothing that prior to September 16 1993 the boundaries shown by the dashed blue and magenta lines had nothing to do with communication requirements.  Rather, if not in a TRSA etc, pilots had to communicate with control towers when within an "airport traffic area", which was a cylinder of airspace 5 statute miles in diameter, extending to 2000' AGL,

(ctd) surrounding all towered airports.  Therefore the Nov 12 92 chart legend only makes sense when we realize that relatively few of the areas charted as Control Zone "extensions"-- which on Sept 16 1993 would be transformed into surface-level Class E "extensions" to Class D (or higher) airspace, a.k.a. E3/E3a/E4 airspaces-- extended closer than 5 statute miles to the airports whose approaches they protected.  The main purpose of the "without communications requirement" phrase in the chart legend was actually to prepare pilots for the changes that would come on September 16, 1993.

This question, or perhaps one of the associated answers, would benefit from having this content integrated into it.

The decision to change the airspace classification in the US to bring it into agreement with ICAO classifications was made in December 1991, as can be seen in the US Library of Congress:  Federal Register: 56 Fed. Reg. 65415 (Dec. 17, 1991).
The ICAO classification was only used from the 16th of September 1993, but as part of the familiarisation process for pilots, the legends of the charts were already updated before.
The Federal Register (FR) contains a table with the Airspace Reclassification Transition timeline.

This table shows that as of October 15, 1992, the first sectional aeronautical charts (SAC), world aeronautical charts (WAC) and terminal aeronautical charts (TAC) are published with legends that indicate both existing and future airspace classifications.
Furthermore the FR specifies that it is anticipated that a segmented magenta line will be used for class E in the future, and that starting October 15, 1992, the new colours will be used in the legends.

An anticipated modification to the symbols on aeronautical charts is
  the addition of a segmented magenta line to represent the controlled
  airspace area for airports without operating control towers that
  extends upward from the surface (Class E airspace).
[....]
The legends in aeronautical charts
  will include both the existing airspace
  classifications and the airspace
  classifications to be effective September
  16, 1993. For example, the solid blue line
  that symbolizes a TCA will be followed
  by "TCA (Class B)." The first charts
  with a dual legend will be published
  October 15, 1992. Commencing
  September 18, 1993, the legends on these
  charts will be reversed (e.g., a solid blue
  line will be followed by "Class B
  (TCA)". Between March 3 and August
  17, 1994, the use of dual indication
  legends will be phased out.

It does relate to it. I've added an additional relevant quote from the FR.

Thank you for the added comment.  It's interesting that as of now, I've seen charts with the dashed magenta line with the legend bearing the old terminology "Control Zone at airport without control tower and Control Zone extension without communications requirement", but none bearing the old terminology AND a reference to "Class E" airspace.

According to this AOPA article, the change was made in Sept, 1993

In a move to simplify airspace classification, the FAA will rename the different airspace categories on Sept. 16, 1993. In order to fall in line with ICAO (International Civil Aviation Organization) practices, it will replace descriptive titles, such as Terminal Control Area, with alphabetical designators, such as Class B Airspace. This change will be phased in gradually. Charts and references will carry both designations until pilots get used to the transition.

https://www.aopa.org/members/ftmag/article.cfm?article=1054

This was a good answer to the other related question, but doesn't really answer this question, as the appearance of the thin dashed magenta line preceded the "alphabet" reclassification, as stated in the question.

Why does it matter when? What are you looking to establish?

Oh, it's a little complicated.  I'm trying to improve a part of my answer to another question, relating to how "control zones" evolved into their present form.

I thought that was just forced on us (in the US) by ICAO requirements.  English is the universal language of aviation, I guess we can live with some ICAO nonsense.  Has been 25 years, I don't really think about it when I fly, as it's always been Class A-B-C-D-E-G for me. I don't fly in A, my plane won't go that high. My home airport is near a B and under its C veil, so radio work is not an issue either.

It appears from the content given in this related answer Re old (1992) US VFR sectional charts-- when was the thin dashed magenta line first used? that it was anticipated that the new markings (e.g. thin dashed magenta line) would be introduced on October 16 1992.  So it is unlikely that the change was made before this date.  Whether US VFR sectional charts ever actually contained legends using the older "Control Zone..." terminology and the newer "Class D", "Class E" terminology is not clear at this point.  Also note the reference to a 28-day AIRAC cycle for issuing new batches of charts, in a comment by "DeltaLima" to this related question Re old (1993) US VFR sectional charts-- when did the term "Class E" first appear?  :"The date will an AIRAC cycle date. Changes to airspace and procedures are only introduced on cycles of 28 days"  As noted in the current question, we know that by Nov 12 the new markings had been introduced-- November 12 is the "effective date" of a sectional chart bearing the new markings .  28 days before November 12 is October 15, before the October 16 target date, so it seems that November 12 1992 may be the answer to the question.  It would be good if someone could locate and check a sectional chart that became effective in mid-October 1992 (presumably October 15 1992) to confirm that the dashed magenta line is absent.


text:
Is there any example of a US VFR sectional chart containing both a reference to "Class E" airspace, and to older terminology for the same airspace?
Is there any example of a US VFR sectional chart whose legend describes the  airspace depicted by the thin dashed magenta line both as "Class E"  or "Class E airspace", and also with the older terminology "Control Zone at airport without control tower and Control Zone extension without communications requirement", or similar?
Based on old sectional charts I've looked at-- and new batches of chart updates become effective on a 28-day cycle-- this would have happened no sooner than 28 days after August 19 1993, and no later than 28 days before November 11 1993.  So it appears that the only possible dates for such a situation are Sept 16 1993 or October 14 1993.
In answering this question please disregard any reference to non-US airspace on the legends of US VFR sectional aeronautical charts.
Note that this related answer to a related question Re old (1992) US VFR sectional charts-- when was the thin dashed magenta line first used? clearly describes an INTENTION to have dual legends (including the old and new airspace terminology) on aeronautical charts between October 15 1992 and mid 1994.  But was this ever actually done on US VFR sectional charts?  Checking charts with effective dates of Sept 16 1993 and Oct 14 1993 should settle the issue.

Sectionals are updated every 6 months, not every 28 days.

Yes any given sectional is updated every 6 months, but it appears that every 28 days another batch of charts--i.e. another batch of locations-- is being revised.  This was pointed out in an answer to another related question.

I've edited the question to try to avoid confusion-- I can see why original wording would have been confusing.

First sentence second paragraph should go "...-- and considering that new batches of chart updates..."

Second-to-last sentence really should read "But-- specifically in relation to airspace marked by the thin dashed magenta line-- was this ever actually done on US VFR sectional charts?"


text:
What is the tallest tower (AGL) identified on a sectional chart within the Class-E-to-surface airspace surrounding an airport in the US?
What is the tallest tower (in terms of AGL) identified on a sectional chart within the Class-E-to-surface airspace surrounding an airport in the US?
For the purpose of the question, "within the Class-E-to-surface airspace surrounding an airport" means within the part of the sectional chart that lies within a dashed magenta line that completely encloses the airport and is not separated from the airport by another dashed magenta or blue line.  I.e. not including within Class-E-to-surface "extensions" that are separate from the Class-E-to-surface that actually surrounds the airport.
For simplicity, let's NOT include Class D or higher that reverts to Class-E-to-surface at night, etc.
"Tower" means any type of structure-- radio antennae, whatever-- that is depicted on the sectional chart with a tower symbol and has the AGL elevation of the top of the tower identified on the sectional chart.

Define "surrounding an airport". Are we talking 200 feet or 20 miles?

within the dashed magenta line-- I will clarify

any type of tower.  I will clarify

Confirm, but I'm changing to 700'.  300' AGL is too easy to find.  Question pertains to using drone to inspect towers but no need to get into that.  No more edits to this question I promise.

At LBL, I see a tower 505 AGL.  That's the best I've found so far.

please do not edit the question in such a way that makes the answers already posted invalid.

Sorry, should have asked a new question.  Oh, I see you rolled it back.  Ok then, new question.

-- Note-- clearly this question has been answered.  I don't think it contributes much to our understanding of anything so I am going to hit the delete button. If moderators feel it should not be deleted then that's fine too, no problem.  Now I see I'm not allowed to-- ok very good.

This question has now been substantially edited in a way that changes its meaning but does not invalidate previous answers.  The edit was in response to suggestions from other users, to allow another question that seemed redundant to be deleted.

@quietflyer edit looks good!

The airport KLBL has a tower marked as 505 AGL.   And another, just barely outside the space at 520 AGL.
That is the highest I've been able to locate.

This seems to be the highest we've found to date.


text:
What is the primary reason that the non-towered airports ACV, TVL, SGU, BIH, and SIT/PASI have E4 extensions to E2 airspace?
NOTE-- this question is not about why it was desired to protect the airspace that ended up within the E4 "extensions".  Rather, this question is about why that was done by designating that airspace as an E4 "extension" rather than simply as part of the E2 airspace that surrounds the airport, which is the more common situation for untowered airports.

What is the primary reason that the (U.S.) non-towered airports ACV, TVL, SGU, BIH, and SIT/PASI all feature E4 Class-E-to-surface "extensions" to the E2 Class-E-to-surface airspace that surrounds the airports?
Why was the airspace not simply all designated as E2 Class-E-to-surface airspace, as was done at SHR?
Is there some established guidance that areas of Class-E-to-surface airspace projecting beyond some given distance beyond the airport whose approaches they protect should be designated as E4 airspace rather than E2 airspace?1  Or is it up to the discretion of the team designing the airspace in each specific case?
There is arguably some regulatory ambiguity around the operational significance of the distinction between E2 airspace and E4 airspace.  Certain FARs pertaining to eligibility for Special VFR clearance, VFR operations under a 1000' ceiling without a Special VFR clearance, ultralight operations, unmanned aircraft operations, aerobatics, and more, might arguably be construed to apply to E2 airspace but not to E4 airspace.  (For more on this, see this related ASE question, and the other related linked questions.)  Was the FAA designing the airspace around these airports with an eye to minimizing the amount of airspace to which these FARs would be applicable, to lift the regulatory burden on certain aviation activities? And/or to limit the amount of airspace in which pilots would be eligible to be granted a Special VFR clearance?
Or was there some other consideration that was the primary reason that the airspace at these airports was designed to have an E2 "Surface Area" with E4 "extensions"?
Bonus question-- why was the airspace portrayed differently on the VFR sectional charts in the first two examples (ACV and TVL) than in the last three examples (SGU, BIH, and SIT/PASI)?

Note that for ACV and TVL, on the depiction on the sectional charts, there is a dashed magenta line separating the rectangular E4 "extensions" from the circular E2 airspace around the airports.
ACV-- California Redwood Coast Humboldt County airport near Arcata CA
Airspace description in
FAA JO 7400.11E-- pp. E-130, E-2122

TVL-- Lake Tahoe Airport, South Lake Tahoe CA
Airspace description in
FAA JO 7400.11E-- pp. E-136, E-2213

Note that for SGU, BIH, and SIT/PASI, on the depiction on the sectional charts, there is no dashed magenta line separating the rectangular E4 "extensions" from the circular E2 airspace around the airports.  Yet in all three of these cases, the rectangular projections are in fact designated as E4 "extensions", as distinct from the E2 inner circles, just as is the case at ACV and TVL:
SGU-- St George Regional Airport, St George UT
Airspace description in
FAA JO 7400.11E-- pp. E-82, E-193

BIH-- Bishop Airport, Bishop CA
Airspace description in
FAA JO 7400.11E-- pp. E-131, E-2134

SIT/PASI-- Sitka Rocky Gutierrez Airport, Sitka, AK
The 2/25/21 Juneau sectional chart portrays a recent airspace change effective 11/5/20 (pictured below)-- the new airspace definitions may be seen here (E2) and
here (E4, after an error was corrected).  The airspace now features a very large E4 "extension".  The old airspace configuration, which at present may still be seen on vfrmap.com, featured an L-shaped area of E2 airspace, with no E4 extensions.

Note that there are other examples of (U.S.) airports that feature large expanses of Class-E-to-surface airspace, with no control tower in the vicinity, where the entire airspace was simply designated as E2 airspace, with no E4 "extensions".  One of the best examples of this is SHR:
SHR-- Sheridan County Airport, Sheridan, WY
Airspace description in
FAA JO 7400.11E-- p. E-91

Footnotes:

E.g. something similar to this guidance regarding E4 extensions to Class D airspace: "If all arrival extensions are 2 NM or less, they will remain part of the basic Class D area. However, if any extension is greater than 2 NM, then all extensions will be Class E airspace."-- from FAA Order JO 7400.2L "Procedures for Handling Airspace Matters".  Also appears as AIM 3-2-5 d1.

All images are clipped from screenshots of the 2-25-2021 editions of the relevant VFR sectional charts, accessed via this FAA webpage.  All are intended to be to same scale.

Note that at TVL, but not at ACV, the chart bears a notation "See NOTAMs/Supplement for Class E(sfc) effective hours", with an arrow pointing specifically to the E4 "extension".  The current Chart Supplement, however, simply states "Airspace: Class E".  To the best of my knowledge, this has been the case ever since the E4 airspace was first created at this location.  The E4 airspace description for TVL in the "Airspace Designations and Reporting Points" document JO 7400.11E does include the note "This Class E airspace area is effective during the specific dates and times established in advance by a Notice to Airmen. The effective date and time will thereafter be continuously published in the Airport/Facility Directory. "  Perhaps there was a desire to facilitate making the E4 extension part-time for some reason in the future, by simply changing the Chart Supplement?  In all the other E2 and E4 airspace descriptions referenced in this question, there is no indication that any of the airspace is intended to be in effect less than 24/7.

The hang gliding symbol within the E2 Class-E-to-surface airspace at BIH is rather "interesting", considering the language of FAR Part 103.

In a comment under a now-deleted answer, I stated "In fact regulatory guidance is given (I can find a reference if needed) that an "extension" is to have the same effective hours as the adjoined E2 area or whatever else the controlled airspace surrounding the airport is."  This question would be improved by providing that specific reference. I'll try to find it again; meanwhile if anyone else knows of it, please drop a note here. Maybe I can find it by the time I need to edit to accommodate the disappearance of the old Sitka airspace configuration from VFRmaps in a few weeks.

Re comment above-- I have not yet found that guidance and am no longer certain that exists-- perhaps I am mis-remembering something.

Re footnote 1, and re "Is there some established guidance that areas of Class-E-to-surface airspace projecting beyond some given distance beyond the airport whose approaches they protect should be designated as E4 airspace rather than E2 airspace?-- guidance as to if and when this practice should be followed, as opposed to just designating the entire area as E2 airspace, seems to be conspicuously lacking from FAA Order 7200.M, "Procedures for Handling Airspace Matters".

Re "The old airspace configuration, which at present may still be seen on vfrmap.com,"-- this is no longer the case; the answer needs editing to reflect this.

The surface-level E4 "extension" has now been dropped at South Lake Tahoe-- see sectional chart at http://vfrmap.com/?type=vfrc&lat=38.894&lon=-119.995&zoom=10&api_key=763xxE1MJHyhr48DlAP2qQ -- the question could use an updating to note that, while still a point of historical interest, the current situation with the airspace near South Lake Tahoe airport is no longer relevant to the question.

Another recent change-- the chart illustration at KACV has been changed to drop the segmented line between the core E2 circle and the E4 "extension"--see http://vfrmap.com/?type=vfrc&lat=40.978&lon=-124.108&zoom=10&api_key=763xxE1MJHyhr48DlAP2qQ -- it seems the FAA has now standardized on this method of depicting E4 "extensions" to E2 airspace in cases where no Class D (or higher) airspace is involved.

New information that will eventually be integrated into answer-- having E4 extensions adjoining E2 airspace (that never converts to Class D or higher airspace) is a relatively recent invention-- the earliest example appears to be at Bishop, 10/12/12, except for South Lake Tahoe, which is an usual case because there is the E4 extensions were added to adjoin Class D airspace effective 12-8-94, and then the tower was closed in 2005 and the Class D airspace was changed to E2-- see https://www.govinfo.gov/content/pkg/FR-1994-06-10/pdf/FR-1994-06-10.pdf

Currently working on a revision to this question, based on changes noted above, and in light of the content here -- https://aviation.stackexchange.com/questions/48103/does-far-91-155c-apply-to-class-e-surface-extensions/55806#55806

Bonus question-- why was the airspace portrayed differently on the VFR sectional charts in the first two examples (ACV and TVL) than in
the last three examples (SGU, BIH, and SIT/PASI)?

After submitting an inquiry through an on-line portal, I recently spoke to a cartographer with the Aeronautical Information Services, who said in his opinion the depiction of the airspace at ACV was a mistake, and should instead resemble the airspace at SGU, BIH, and SIT/PASI.  He is taking steps to make the change, and expects it to go into effect for the next edition following the 2-25-2021 edition.  TVL presents a more complex case because the airspace description in J.O. 7400.11E does indicate that the E4 extension may not always be in effect 24/7, even though the present Chart Supplement does indicate that all the Class E airspace at this location is in effect 24/7.  The cartographer indicated that he would take steps to see if there should be a change in how the airspace at TVL is depicted as well.

Was the FAA designing the airspace around these airports with an eye
to minimizing the amount of airspace to which these FARs would be
applicable, to lift the regulatory burden on certain aviation
activities?

This seems unlikely.

Was the FAA designing the airspace around these airports with an eye
to minimizing the amount of airspace to which these FARs would be
applicable, to... limit the amount of airspace in which pilots would
be eligible to be granted a Special VFR clearance?

This seems unlikely too.  While it doesn't speak directly to the intent of the airspace designers, it is the case that in early 2019, I called the ARTC Centers controlling the E2 and E4 airspaces around these airports (except SIT/PASI which did not yet have E4 airspace at that time) and in every case the staffer I spoke to told me that in actual practice, they controlled both the E2 airspace and the E4 airspace.  The staffers also told me that they made no distinction between E2 and E4 airspace at these locations in terms of granting a SVFR clearance.

What is the primary reason that the (U.S.) non-towered airports ACV,
TVL, SGU, BIH, and SIT/PASI all feature E4 Class-E-to-surface
"extensions" to the E2 Class-E-to-surface airspace that surrounds the
airports?
Why was the airspace not simply all designated as E2
Class-E-to-surface airspace, as was done at SHR?

This answer is unable to address those aspects of the question at this time; it remains unknown.
But, regardless of the reason, it is a positive thing for those flying small Unmanned Aircraft under Part 107 or the 2018 "Recreational Exception", and arguably for ultralight and aerobatic fliers as well.  For more, see this related ASE answer.

@DeltaLima -- quite some time ago you stated "I am interested to hear about what you find out. Give me a ping when you do!" -- the first paragraph of this answer may be of interest.

@Devil07 -- in a comment quite some time ago you said "Perhaps, ACV is a cartographic typo (it does happen), and TVL is because it has different effective hours" -- that seems to be the answer (see first paragraph of this answer.)  At least, TVL has the appearance of the possibility of having different effective hours. When the cartographers see that in the airspace description, they probably feel forced to indicate it on the chart. However as noted in a comment under question, it seems that the E4 really shouldn't ever be allowed to have different hours from the E2. So some mystery remains.

The surface-level E4 "extension" has now been dropped at South Lake Tahoe-- see sectional chart at http://vfrmap.com/?type=vfrc&lat=38.894&lon=-119.995&zoom=10&api_key=763xxE1MJHyhr48DlAP2qQ --

Another recent change-- the chart illustration at KACV has been changed to drop the segmented line between the core E2 circle and the E4 "extension"--see http://vfrmap.com/?type=vfrc&lat=40.978&lon=-124.108&zoom=10&api_key=763xxE1MJHyhr48DlAP2qQ -- it seems the FAA has now standardized on this method of depicting E4 "extensions" to E2 airspace in cases where no Class D (or higher) airspace is involved.

The original edition of the "Airspace Designations and Reporting Points" document clearly did not contemplate that there would be any E4 "extensions" to core E2 "surface areas", except in cases where those E2 "surface areas" converted to Class D (or higher) airspaces during the hours that a control tower was open.  This information will eventually be incorporated into this answer.

Currently working on a revision to this answer, based on changes noted above, and in light of the content here -- https://aviation.stackexchange.com/questions/48103/does-far-91-155c-apply-to-class-e-surface-extensions/55806#55806

The extensions are aligned with the runway for instrument approaches.
The following is an excerpt from the Aeronautical Information Manual (AIM,
Chap 3. Airspace, Section 2. Controlled Airspace)

e. Functions of Class E Airspace. Class E
  airspace may be designated for the following
  purposes:

Surface area designated for an airport
  where a control tower is not in operation. Class E
  surface areas extend upward from the surface to a
  designated altitude, or to the adjacent or overlying
  controlled airspace. The airspace will be configured
  to contain all instrument procedures.
(a) To qualify for a Class E surface area, the
  airport must have weather observation and reporting
  capability, and communications capability must exist
  with aircraft down to the runway surface.
  (b) [...]
[...]
Airspace used for transition. Class E
  airspace areas may be designated for transitioning
  aircraft to/from the terminal or en route environment.
(a) Class E transition areas extend upward
  from either 700 feet AGL (shown as magenta vignette
  on sectional charts) or 1,200 feet AGL (blue vignette)
  and are designated for airports with an approved
  instrument procedure.
  (b) The 700-foot/1200-foot AGL Class E
  airspace transition areas remain in effect continuously,
  regardless of airport operating hours or surface
  area status.

This answer seems to overlook the opening sentence in the question-- "NOTE-- this question is not about why it was desired to protect the airspace that ended up within the E4 "extensions". Rather, this question is about why that was done by designating that airspace as an E4 "extension" rather than simply as part of the E2 airspace that surrounds the airport, which is the much more common situation for untowered airports."

Also, the illustration doesn't really show an "extension", at least not in the same sense the question was asking about, which referred to a rectangular projection of airspace that is separated (on the chart depiction) by a dashed line from the main chunk of controlled-to-surface airspace that actually surrounds the airport.


text:
Why are KACV and KTLV depicted differently on sectional charts than KSGU, KBIH, and KTPL, even though each is E2 w/ E4 extensions?
This question specifically is about non-towered airports surrounded by round circles of E2 airspace, with one or more "extensions" of E4 airspace that abut the E2 airspace.  Therefore it is not the same as the related question that may be noted above.  Also the question noted above is about how certain airspace functions when the tower is open versus closed, not about why it is drawn a certain way on the sectional chart.
Note that this configuration-- a non-towered airport surrounded by a round circle of E2 airspace, with one or more "extensions" of E4 airspace that abut the E2 airspace -- is a somewhat unusual airspace configuration. In the vast majority of cases where a non-towered airport is surrounded by Class-E-to-surface airspace that has one or more rectangular projections, the entire Class-E-to-surface airspace is designated as E2 airspace.  No E4 "extensions" are designated.  Examples: KONP, KSHR.  This question is not about that airspace configuration.

The question:
According to the FAA's "Airspace Designations and Reporting Points" document Order 7400.11C ( https://www.faa.gov/documentLibrary/media/Order/JO_7400.11C.pdf ) , KACV, KTLV, KSGU, KBIH, and KTPL all share the same airspace configuration: a round E2 circle around the airport, with one or more E4 "extensions".
Why then on the aeronautical sectional charts is the Class-E-to-surface airspace near KACV and KTLV depicted differently than the Class-E-to-surface airspace near KSGU, KBIH, and KTPL?
The first two are depicted with a dashed magenta circle going all the way around the airport, while the last three are not.  In the case of the last three the charts give the impression that all the Class-E-to-surface near the airport may be E2, with no E4 "extensions".
KACV: California Redwood Coast Humboldt County near Arcata -- http://vfrmap.com/?type=vfrc&lat=40.978&lon=-124.108&zoom=10
E2 described on page E-127 of the "Airspace Designations and Reporting Points" document, E4 described on page E-211
KTLV South Lake Tahoe --
http://vfrmap.com/?type=vfrc&lat=38.894&lon=-119.995&zoom=10
E2 described on page E-135, E4 described on page E-220
KSGU -- St. George Regional Airport, UT --
http://vfrmap.com/?type=vfrc&lat=37.036&lon=-113.510&zoom=10
E2 described on p. E-84, E4 described on p. E-190
KBIH -- Bishop Airport, Bishop CA --
http://vfrmap.com/?type=vfrc&lat=37.373&lon=-118.364&zoom=10
E2 described on p. E-130, E4 described on p. E-211-212
KTPL -- Temple, Draughon-Miller Central Texas Regional Airport, TX --
http://vfrmap.com/?type=vfrc&lat=31.152&lon=-97.408&zoom=10
E2 described on on p. E-127, E4 described on p. E-209
It seems that it would be good for the chartmakers to standardize on the notation used at KACV and KTLV rather than the notation used at KSGU, KTPL, and KBIH, since there appear to be some functional differences between the E2 and E4 airspaces, as explored in the related question linked below.
Related question with links to additional related content --
What is the primary reason that the non-towered airports KACV and KTLV have E4 extensions to E2 airspace?

Possible duplicate of Can the Class-E-to-surface airspace at Eagle County Regional airport KEGE be ignored when the tower is open?

Dashed Class E is about protecting approaches.

It's not a duplicate because KEGE is a towered airport and towered airports almost always have a different configuration to the Class-E-to-surface airspace than KEGE.  KEGE is an oddball and doesn't really shed light on the present question

Class E is not about tower/no-tower. KEGE has a precision approach and that is what the Class E protects.

Again, no relation to tower or tower service, just approaches.

The point that I am trying to make is that since Class E's respond to approach configurations and other issues, there is no reason to expect rhyme nor reason in what they look like, etc. That is why there is no "unusual" configuration for these. They respond to the circumstances of what the FAA wants to achieve at that particular location. What the chart is depicting says that the airports do not have the same airspace configuration.

This question is now answered here https://aviation.stackexchange.com/a/84358/34686 .  Actually, I forgot the present question existed, when I added the "bonus question" part to the other related question.

Perhaps I ought to delete the present question?

Except that someone else has offered an answer, so... ?

Or I could edit https://aviation.stackexchange.com/questions/63970/what-is-the-primary-reason-that-the-non-towered-airports-acv-tvl-sgu-bih-and to remove the part that duplicates this question--

But I really think https://aviation.stackexchange.com/questions/63970/what-is-the-primary-reason-that-the-non-towered-airports-acv-tvl-sgu-bih-and is a better question with the new addition.  Guess I won't do anything for right now, until someone insists.

PS the airspace at KTPL has now been reconfigured to a smaller size, and there is no longer any E4 "extension" airspace there.

More recent changes-- the airspace at KTLV has been reconfigured to drop the extension-- see http://vfrmap.com/?type=vfrc&lat=38.894&lon=-119.995&zoom=10&api_key=763xxE1MJHyhr48DlAP2qQ.  Also, the chart illustration at KACV has been changed to drop the segmented line between the core E2 circle and the E4 "extension"--see http://vfrmap.com/?type=vfrc&lat=40.978&lon=-124.108&zoom=10&api_key=763xxE1MJHyhr48DlAP2qQ -- it seems the FAA has now standardized on this method of depicting E4 "extensions" to E2 airspace in cases where no Class D (or higher) airspace is involved.

It looks like the answer is "it's a mistake or inconsistency in the sectional charts".  Whether or not it's an outright mistake is debatable, but it certainly appears to be an inconsistency, and one can argue that the depiction used for KACV and KTLV provides some useful information that the depiction used for KSGU, KBIH, and KTPL does not.
It is the case that at KTLV-- but not at KACV or the other 3 airports mentioned here-- the description of the E4 airspace in the "Airspace Designations and Reporting Points" document has a note to see the A/FD for effective hours, while the E2 airspace has no such note.  The sectional chart has a similar notation, with an arrow pointing specifically to the E4 airspace.  (It also is the case that the current A/FD entry for KTLV simply reads "Airspace: Class E".)  It seems possible that the policy of the charting agency is to depict the adjoined E2 and E4 Class-E-to-surface areas as two distinct areas in any case where there is a possibility why their effective hours may be different.  If this is true, it doesn't explain why that style of depiction was also used for KACV.

If the bumpout is fully enclosed by the magenta line, then it is part of the "Class E airspace designated for an airport". If it is truly an extension, then it is outside the main circle (which is usually a Class D or higher, but can be Class E) and is not part of the Class E airspace designated for the airport. Extensions have different rules for when they revert down to Class G, and different rules for special VFR and whether UAS need permission to fly in them (they don't). Big topic of discussion in the drone world.
http://goldsealgroundschool.com/uav-library/surface-E-authorizations.pdf
EDIT: there does appear to be a conflict between the charts and docs on this one. Not the first time...

What you wrote in your answer is exactly what is the normal practice and exactly what I thought was always the case until about a week ago.  However, if you look closely at the "Airspace Designations and Reporting Points" document linked in the question, you'll find that at KSGU, KBIH, and KTPL, the airspace in the rectangular parts of the Class-E-to-surface airspace is actually designated as E4 not E2, even though it is FULLY ENCLOSED by the very same magenta line that encloses the airport, with no separation.  If you go look at the FAA interactive sUAS map you'll see you need no (ctd)

(Continuing) If you go look at the FAA interactive sUAS map ( https://faa.maps.arcgis.com/apps/webappviewer/index.html?id=9c2e4406710048e19806ebf6a06754ad ) you'll see you need no special permission to fly in the part of the Class-E-to-surface that in the rectangular part of the shape at these three airports, even though you'd never know it by looking at the sectional charts.  That's basically what this question is all about, among other things.

I see what you mean...I've worked with the FAA to change a Class E Surface airspace, and wouldn't be surprised if there was simply an error here. They juggle a lot of space.

I'd give one of their airspace guys a call and ask.

You mean an error on the interactive map ?  Maybe but the thing is the interactive map exactly matches the FAA's "Airspace Designations and Reporting Points" document Order 7400.11C which is the gold standard.  The depiction on the sectional charts is the red herring here it seems.

I agree the sectional looks out of sync. An interesting question, since the FAA stuff is the law, is which takes precedence when a conflict arises? Chart or airspace docs?

Not the chart!  The documents

Curious what airspace designation you were able to get changed.  If you care to chat a bit more I'll send you a message from a room I just created.

Let us continue this discussion in chat.

MikeY, do you possibly want to consider deleting your answer to this question?  You could save to clipboard and then repost to the other related question, if you wished.  The reason I'm asking is that I'd really prefer to delete this question since I've forgetfully gone and essentially added it to the related question https://aviation.stackexchange.com/questions/63970/what-is-the-primary-reason-that-the-non-towered-airports-acv-tvl-sgu-bih-and , but I don't want to delete a question with someone else's answer attached.  It's up to you, no problem either way.  If you are still visiting ASE...


text:
What are all the US airports where E4 "extensions" change to E2 and become part of a larger E2 "surface area" when the tower closes for the night?
Of all the US airports with part-time control towers and with E4 "extensions", in which cases are the airspace definitions written so that the E4 "extensions" change to E2 and become part of a larger E2 "surface area" -- which also includes the airspace that was Class D during the daytime -- when the tower closes for the night?

With these 21 airports,  the airspace definitions are written so that the E4 "extensions" change to E2 and become part of a larger E2 "surface area" - which also includes the airspace that was Class D during the daytime -- when the tower closes for the night.
The page number references are for the FAA's "Airspace Designations and Reporting Points" document, FAA JO 7400.11C.  The page numbers listed are for the E4 airspace designation, the E2 airspace designation, and Class D airspace designation.

KENA or PAEN
AAL AK Kenai, AK
Kenai Municipal Airport
E-158, E-7, D-5
KGCK
ACE KS Garden City, KS
Garden City Regional Airport
E-159, E-17, D-7
KJEF
ACE MO Jefferson City, MO
Jefferson City Memorial Airport
E-162, E-21, D-11
KNHK
AEA MD Patuxent River, MD.
Patuxent River NAS (Trapnell Field)
E-165, E-28-29, D-15
KITH
AEA NY Ithaca, NY
Ithaca Tompkins Regional Airport, Ithaca
E-167, E-30-31, D-19
KPOU
AEA NY Poughkeepsie, NY
Dutchess County Airport, Poughkeepsie
E-167, E-32, D-19-20
KCXY
AEA PA Harrisburg, Capital City Airport, PA
Capital City Airport, Harrisburg
E-169, E-35, D-22
KLNS
AEA PA Lancaster, PA
Lancaster Airport
E-170, E-35, D-23-24
KRDG
AEA PA Reading, PA
Reading Regional/Carl A. Spaatz Field, Reading
E-170, E-36, D-24
KUNV
AEA PA State College, PA
University Park
E-171, E-35, D-25
KIPT
AEA PA Williamsport, PA
Williamsport-Lycoming County Airport
E-171, E-37, D-24
KLYH
AEA VA Lynchburg, VA
Lynchburg Regional-Preston Glenn Field Airport
E-172, E-38-39, D-26
KNTU
AEA VA Oceana NAS, VA
Oceana NAS (Apollo Soucek Field)
E-172, E39-40, D-28
KAPN
AGL MI Alpena, MI
Alpena County Regional Airport
MI E-176, E-45, D-36
KMKG
AGL MI Muskegon, MI
Muskegon County Airport
E-177-178, E-48, D-39
KRST **
AGL MN Rochester, MN
Rochester International Airport
E-178, E-54, D-41
KSTC **
AGL MN St. Cloud, MN
St. Cloud Regional Airport
E-178, E-54-55, D-41
KGFK
AGL ND Grand Forks, ND
Grand Forks International Airport
E-179, E-56-57, D-42-43
KILN **
AGL OH Wilmington, OH
Wilmington, Wilmington Air Park
E-181, E-59, D-46
KRAP **
AGL SD Rapid City, SD
Rapid City Regional Airport
E-181, E-61, D-47
KPRC
AWP AZ Prescott, AZ
Ernest A. Love Field
E-210, E-129, D-140

** means that in these 4 cases, no limit on the effective hours is given for the E4 airspace - apparently it is simply assumed that the E4 will be superseded by the E2 whenever the E2 is in effect.
Among airports with part-time control towers and with E4 "extensions", the style of airspace designation that is much more common over the nation as a whole is for the E4 airspace to be in effect 24/7, and for only the Class D airspace to change to E2 when the tower closes for the night.  Here's an example of such a designation:
KMFR
ANM OR Medford, OR
Rogue Valley International-Medford Airport
E-189, E-80-81, D-64
Note that there is no way to tell one style of designation from the other just by looking at the sectional chart or the Airport/ Facility Directory.  Perhaps this may be taken as a hint that the distinction rarely matters in actual practice.
Note the uneven distribution of the airports in the list above among states and regions.  For some reason, in some regions such as the northeast, the style of airspace designation that we're discussing here is actually more common than the style that we see at KMFR.
To understand why the distinction between the two styles of airspace designation might matter, at least in theory, to pilots flying at first light before the tower is open for the day, including ultralight flight, aerobatic practice, non-recreational sUAS (drone) activities, and possibly now that the FAA is implementing some new regulatory changes, recreational sUAS (e.g. RC model airplane) activities, see the first two links below.  Whether this distinction has ever yet mattered in actual practice is an open question-- perhaps in actual practice the airspace in these extensions is simply treated as E4 airspace 24/7 in relation to FARs 91.303c, 103.17, and 107.41.  Nonetheless, pilots of such aircraft would undoubtedly hope that this unusual style of airspace designation is avoided in the future.
It is interesting to note that on the FAA's LAANC map for authorization for sUAS (drone, model airplane) flight, these projecting E4 extensions that change to E2 when the tower is closed at the 21 airports listed above are NOT covered by the grid squares that indicate the areas where pre-authorization is required.  This supports the idea that these extensions are simply treated as E4 airspace 24/7, at least in relation to FAR 107.41.
Unlike what is implied by the actual language of FAR's 91.303c and 9.155c, it seems unlikely that the distinction between the two different styles of airspace designation has ever mattered in actual practice to a pilot seeking to obtain a Special VFR clearance, or planning to fly beneath a cloud ceiling below 1000' AGL without obtaining a Special VFR clearance.  See the third and fourth link below for more.
For more, see these related question or answers:

Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization?
What is the primary reason that the non-towered airports KACV and KTLV have E4 extensions to E2 airspace?
In actual practice, do ARTC Center personnel generally follow the rule that a SVFR clearance may be granted in E2 airspace but not E4 airspace? (US)
In the US, is there much variation among the various ARTC Centers as to where a SVFR clearance may be granted?
Does an SVFR clearance extend to Echo surface extensions?
Does an SVFR clearance extend to Echo surface extensions?
Does FAR 91.155c apply to class E surface extensions?

"Whether this distinction has ever yet mattered in actual practice is an open question-- perhaps in actual practice the airspace in these extensions is simply treated as E4 airspace 24/7 in relation to FARs 91.303c, 101.33a, 103.17, 107.41." could be the subject of another question, but I'll save it for another day.

See this related ASE answer https://aviation.stackexchange.com/questions/54048/what-class-is-the-airspace-at-kgcn-when-the-tower-is-closed/75377#75377 to learn an interesting tidbit about the inconsistent way in which the 21 airports listed above are noted on sectional charts

The E4 "extension" at KILN has now been removed.

The double asterisks need to be added to KMKG, KITH, KPOU, KLNS (changed 11/08/18), KRDG (changed 11/08/18), KIPT (changed 11/08/18), KLYH (changed 11/08/18), and KAPN.

The surface-level Class E "extensions" have now been entirely removed at KPRC in Prescott AZ, KMKG in Muskegon MI, and KILN in Wilmington OH.

In the list of links at the end, the question "In actual practice, do ARTC Center personnel generally follow the rule that a SVFR clearance may be granted in E2 airspace but not E4 airspace? (US)" has been deleted from ASE, and the title of "In the US, is there much variation among the various ARTC Centers as to where a SVFR clearance may be granted?" has been changed.  It's now https://aviation.stackexchange.com/q/64205/34686